1. Field of the Invention
This invention concerns a method for forming crystalline semiconductor layers and a method for fabricating thin film transistors, active matrix liquid crystal devices and solar cells that employ these crystalline semiconductor layers.
2. Description of the Related Art
Polycrystalline silicon and other semiconductor films are used widely in thin film transistors ("TFT" in the specifications of this application) and solar cells. The performance of these semiconductor devices is strongly dependent on the quality of the semiconductor layer, which wholly constitutes the active portion of the semiconductor device. Needless to say, if a high-quality semiconductor layer can be created, a semiconductor device with correspondingly high performance can be produced. For example, in a polycrystalline silicon thin film transistor (poly-Si TFT) used in such products as liquid crystal display devices, the higher the quality of the polycrystalline silicon (poly-Si) layer, the faster the switching speed of the TFT. Likewise, given roughly similar light absorption efficiency, a solar cell having a semiconductor layer with a higher degree of crystallinity will convert energy more efficiently. There is, therefore, strong demand in many industries for high quality crystalline semiconductor layers.
However, the formation of this kind of high quality semiconductor layer is generally difficult to achieve and, moreover, is subject to important limitations. In the field of TFTs, polycrystalline silicon layers having relatively high mobility are formed by fabricating transistors by means of a high temperature process where maximum process temperatures reach about 1000.degree. C. Because of this, semiconductor films and semiconductor devices can be formed only on those substrates having thermal resistance properties that enable them to withstand high temperature processing. For this reason, all of today's poly-Si TFTs are formed on expensive, small quartz glass substrates. For the same reason, amorphous silicon (a-Si) is normally used for solar cells.
Against this backdrop, various research has been conducted on methods of forming high quality semiconductor layers at the lowest temperature possible. Solid-phase crystallization is known as the first such method. In this process, an a-Si film is formed on the substrate and then subjected to annealing at a temperature of approximately 600.degree. C. for a minimum of 10 hours, thereby converting said a-Si film to a poly-Si layer. Laser crystallization is acknowledged to be the second process. In this method, an a-Si film is first deposited and then exposed to laser irradiation, thereby promoting crystallization of the silicon film.
However, the first of the conventional technologies (solid phase crystallization) requires annealing over a long period of time--more than 10 hours--and thus suffers from extremely poor throughput. Moreover, in this process thermal deformation of the substrate arising from prolonged heating of the entire substrate has become a major problem, meaning essentially that inexpensive, large glass substrates cannot be used. The problem with the second of the conventional technologies (laser crystallization) is that crystallization does not progress if the laser irradiation energy is too low, while high energy will damage the semiconductor film. Hence, satisfactory, high quality crystalline films cannot be produced under either of the irradiation conditions. Moreover, extensive nonuniformity in crystallinity is known to occur with each laser irradiation. The result is that even if these semiconductor films are applied in TFTs, for example, good transistor characteristics cannot be obtained.
Accordingly, a third method is being studied, which combines the second of the conventional technologies (laser crystallization) with a variation of the first (furnace annealing). This is a semiconductor film annealing process that is performed after the semiconductor film is crystallized by a laser. In this process the annealing temperature is lower (450.degree. C. to 550.degree. C.) than that used in solid-phase crystallization and the annealing time is shorter (one to five hours). Nevertheless, this process, too, has essentially the same problems as those of the first method. That is, even if the annealing temperature were kept to about 450.degree. C., throughput would be poor because annealing would be required for at least several hours and, moreover, thermal distortion of the substrate could not be ignored.
Therefore, the aim of the present invention is to solve the aforementioned problems. The objective is to provide a method of forming high quality crystalline semiconductor layers with high throughput without subjecting the substrate to excessive thermal stress and, using this method, to provide a method of producing high performance thin film transistors and solar cells.